We have investigated whether telomeres are prone to oxidative DNA damage. In comparison to non-telomeric DNA, telomeric DNA harbors more oxidative base lesions in both human and mouse cells, which is likely due to inefficient DNA repair at telomeres. Because the base excision repair (BER) pathway is primarily responsible for the removal of non-bulky oxidative base lesions and because telomeres are unique in their sequences, binding proteins, and DNA structures, we therefore examined if these unique telomere factors affect oxidative base repair by BER at telomeres. We examined several telomere specific factors including telomere repeat sequences, telomere binding proteins (TRF1 and TRF2) and an in vitro-assembled T-loop, and none of the factors appear to affect BER at telomeres in vitro. Previously we have examined the impact of oxidative guanine base lesions at telomeres in budding yeast and mice. We also found that a key BER protein, Ogg1 DNA glycosylase is critical in oxidative guanine base repair. Because oxidative stress can also cause oxidative modifications at pyrimidine bases, we have investigated if oxidative pyrimidine damage could accumulate at telomeres. We employed a mouse model with a loss in the Nth1 DNA glycosylase that excises oxidized pyrimidines. Nth1 deficiency leads to an increase in oxidative pyrimidine lesions at telomeres in mouse tissues and primary cells, suggesting that Nth1 is critical in repairing oxidative pyrimidine damage in telomeric DNA in mammalian cells. Ablation of Nth1 function results in telomere attrition and telomere strand breaks that are dependent on an environmental oxidative exposure. These results suggest that Nth1 plays an important role in telomere base repair and telomere maintenance against oxidative stress-induced DNA damage. We have begun to characterize other key base excision repair proteins, including Xrcc1 in telomere maintenance in mice. Our long term goal is to investigate the role of oxidative base damage and BER deficiency in telomere damage-induced cellular senescence and organismal aging, particularly using mouse strains that mimic humans with regard to telomere length and telomerase activity. The will serve as useful guidelines for future studies that employ human cells. Besides these major activities, we have characterized DNA repair protein net work that associates with telomeres. We are investigating the roles of DNA repair proteins, e.g. Fanconi anemia proteins and a helicase protein RecqL4, in regulating telomere T-loop and telomere length in collaboration with other laboratories. Our preliminary results indicate that deficiency in these DNA repair proteins can lead to changes in telomere length, telomere replication, and telomere recombination, which ultimately impact telomere function.